In this type of technical field, the next generation mobile communication schemes, which may become successors of W-CDMA and HSDPA, are being discussed by standardization group 3GPP. Typical ones of the next generation mobile communication systems are a LTE (Long Term Evolution) system and an IMT advanced system (the fourth generation mobile communication system). Summary of the LTE is described in non-patent document 1, for example.
In the LTE, one or more physical channels in both uplinks and downlinks are shared among multiple mobile stations (user equipments) for communications. The channel shared among multiple mobile stations is generally called a shared channel and is referred to as a PUSCH (Physical Uplink Shared Channel) for the uplinks and a PDSCH (Physical Downlink Shared Channel) for the downlinks. Also, transport channels mapped to the PUSCH and the PDSCH are referred to as an Uplink-Shared Channel (UL-SCH) and a Downlink-Shared Channel (DL-SCH), respectively.
In a communication system using the shared channels, it is necessary to signal which mobile stations are assigned the shared channels for each subframe. This assignment of radio resources is called scheduling. The subframe may be of 1 ms, for example, (although other values may be used) which may be called a TTI (Transmission Time Interval). Control channels used for the signaling are referred to as a PDCCH (Physical Downlink Control Channel) in the LTE. The PDCCH may be referred to as a Downlink L1/L2 control channel, a DL L1/L2 control channel or DCI (Downlink Control Information). The PDCCH may include a DL/UL Scheduling Grant and/or a TPC (Transmission Power Control) bit, for example.
More specifically, the DL scheduling grant may include downlink resource block assignment information, a user equipment (UE) ID, the number of streams, information on precoding vectors, information on a data size and a modulation scheme, information on HARQ (Hybrid Automatic Repeat reQuest) and/or others, for example. The DL scheduling grant may be referred to as DL assignment information, DL scheduling information or others.
Also, the UL scheduling grant may include uplink resource block assignment information, a user equipment (UE) ID, information on a data size and a modulation scheme, uplink transmit power information, information on demodulation reference signals, a CQI request bit and/or others, for example.
The scheduling is conducted depending on quality of channel state. The uplink channel state can be measured at a base station based on reception quality of reference signals (or pilot channels) transmitted from a user equipment. The downlink channel state is measured at the user equipment, and the measurement result is reported to the base station. Specifically, the base station transmits a reference signal (or a pilot channel) in downlinks, and the user equipment measures the downlink channel state based on the reception quality of the reference signal. The downlink channel state may be represented as a channel quality indicator (CQI) resulting from quantization of a quality measurement value such as a reception level or SINR. The CQI may be referred to as CSI being a generic indicator of the CQI, a PMI (Pre-coding Matrix Indicator) and a RI (Rank Indicator).
Generally, there are two methods of reporting the CQI to the base station. In the first method, a dedicated control resource is assigned to a user equipment in advance, and the CQI is periodically reported without explicit indication from the base station. The dedicated control resource is referred to as a PUCCH (Physical Uplink Control Channel). If uplink transmission of a shared channel is allowed, the CQI may be transmitted in the PUSCH instead of in the PUCCH.
In the second method, the CQI s reported depending on explicit indication from the base station. As stated above, the uplink scheduling grant includes the CQI request bit, and the CQI request bit is indicated to the user equipment through transmission of the uplink scheduling grant. If the CQI request bit is equal to a predefined value (e.g., “1”), the allowed uplink shared channel is used to report the CQI. In other words, the allowed uplink shared channel is used to transmit both the CQI and traffic data. On the other hand, if the CQI request bit is not equal to the predefined value (e.g., “0”), the allowed uplink shared channel is used to transmit user's traffic data. The second method is preferable over the first method for the case where a large amount of information is reported at one time. The method of using the CQI request bit to indicate necessity of reporting the CQI to the user equipment is described in non-patent document 2, for example.